When two asynchronous devices communicate via serial dam streams, some mechanism must be used to delineate the start and end of individual pieces of information commonly referred to as frames of information. Such a mechanism or protocol should provide four basic functions for each communicating device:
1) achieve frame alignment; PA1 2) detect alignment errors; PA1 3) notify the other device if an alignment error is detected; and PA1 4) recover from alignment errors.
The two most prevalent dam-link communication protocols that include a framing mechanism are ISO Standard DIS 3309.2 (HDLC) and ANSI Standard T1.107 (CCITT G.733). Each of these standards and their variations are insufficient with respect to achieving the mechanism presented above.
Variations of HDLC include SDLC, LAPB, and LAPD. All of these protocols transmit dam as a large block with framing overhead appended to the beginning and end of the block. These protocols effectively establish frame alignment prior to each transmission, unlike the present invention. Moreover, these protocols do not detect alignment errors until the entire block of information has been transmitted. The present frame alignment method and system does not reestablish frame alignment for each transmission, but rather only performs a frame alignment when a frame alignment error is detected. The frame alignment error is detected shortly after it occurs, allowing for faster recovery from the error. The present invention also contains less framing overhead for data transfers less than 122 bytes.
In addition, by breaking the transmitted data into smaller frames during transmission, the present invention allows a logical connection to be established or disestablished in the opposite direction of transmission, providing a true full-duplex communications path. Because HDLC sends data in a contiguous block, it is unable to send control information relating to data reception while the block is being transmitted.
The ANSI T1.107 and the CCITT G.733 standards are commonly referred to in the art as "T1" standard in the United States. The T1 carrier system is a time division multiplex system for digitized information, and has frame alignment information contained in a constant serial bit stream. Although the framing information overhead is low, there is no built-in error notification protocol. Moreover, there is no mechanism to stop the physical transmission when there is no data to transmit. The present frame alignment method and system is able to only transmit when there is data to send, thereby reducing power consumption and system noise.
Thus, none of the prior art communication standards combine error notification and frame alignment, wherein frame alignment is both fast and efficient. The prior art also does not combine such a protocol with true full-duplex communication and low power consumption, wherein the associated clock may be disabled between valid communications.